Several recent reports have demonstrated that endothelial cells can differentiate into mesenchymal cell types [1-8], a process called endothelial-mesenchymal transition (EndoMT or EndMT). In turn, endothelial cell-derived mesenchymal cells are capable of differentiating into multiple cell types, and have been shown to form fat, cartilage, and bone, respectively[8]. Recent studies indicate that EndMT contributes to disease progression in post-developmental tissue, and thorough investigations have shown roles in fibrosis[1, 3-5], cancer[2], and heterotopic ossification associated with fibrodysplasia ossificans progressiva (FOP) [8]. Therefore, the factors that influence EndMT may be of particular relevance in understanding and manipulating disease progression. The extracellular matrix (ECM) provides a wide variety of biochemical and biophysical cues that guide cell function, and several matrix components have been implicated in EndMT during tissue development, including proteoglycans [9], vitronectin, collagen, fibronectin and laminin [10-11]. In addition, TGF-b superfamily growth factors can induce EndMT[3, 8] and the influence of soluble TGF-b on endothelial cell behavior is dependent on ECM components present in culture [12]. Further, biophysical factors are also known to be important determinants of endothelial cell behavior, mesenchymal cell differentiation [13], fibrosis, and bone disease [5, 14-24]. Taken together, these previous studies implicate ECM-derived signals as likely regulators of EndMT and, in turn, disease progression. However, the role for the ECM in guiding EndMT is a largely unexplored area of research, perhaps due to the relatively recent discovery that this process occurs in adult tissue. Here, we propose an approach to identify ECM-mimetic signals that influence EndMT. The experimental approach is based on our experience creating materials to efficiently control cell-material interactions. The guiding hypothesis of the application is that biochemical and biophysical factors that mimic extracellular matrix properties will each significantly influence the process of EndMT. The proposed studies are Significant, as they will provide critical new insights into how the extracellular matrix contributes to EndMT, and the diseases in which it is implicated. The few factors that are known to influence EndMT in adult tissue (e.g., BMP-4, TGF-b1, TGF-b2) and the changes in endothelial cell phenotype that result (e.g., myofibroblast activation, heterotopic osteogenesis) are generally implicated in a range of diseases that are of substantial and broad relevance. Therefore, a better understanding of the role of EndMT could have a substantial benefit for human health. The proposed studies are Innovative, as they use novel, chemically-defined cell culture arrays to efficiently probe the effects of ECM-mimetic signals. This approach for probing ECM-mimetic signals is ideal in the case of the EndMT process, as the ECM plays a poorly understood but likely critical role in EndMT. Further, understanding how synthetic biomaterials influence EndMT could ultimately have relevance to medical implant design, particularly in scenarios in which it is important to limit fibrosis or heterotopic ossification.